1. Field of the Invention
The present invention relates to a novel process for the preparation of keto compounds from alkali metal salts via electrodialysis.
The keto compounds prepared according to the invention method are valuable intermediates in the preparation of, for example, heterocyclic compounds, pharmaceuticals, plant protection agents and aroma substances.
2. Discussion of the Background
Ester condensation can be used to convert carboxylic esters to dicarbonyl compounds. Using this method, ethyl acetoacetate can be prepared from ethyl acetate. Lactones, too, can be converted using ethyl acetate into the corresponding acetyl derivatives. Because of the relatively low reactivity of the ester carbonyl group, however, strong bases such as alkali metal alcoholates are required as condensing agents. These compounds can be prepared by reacting an alcohol with the alkali metal or with alkalis. The alcoholates can also be prepared in an electric field, for example by the amalgam method or in accordance with DE-A-42 33 191 by electrodialysis.
The ester condensation of lactones with carboxylic esters is one method for the preparation of acyl lactones. Owing to the sensitivity of many lactones the condensing agents preferably used, in accordance with Angew. Chemie 71, 709-752, (1959), are sodium, potassium, Na methylate, Na hydride, Na amide, diisopropylmagnesium bromide and triphenylmethylsodium.
To prepare 2-acetylbutyrolactone from 7-butyrolactone and ethyl acetate, processes have been developed in which sodium, NaH and Na alcoholates (JP 70/009 538) are employed as condensing agents. A further process (JP 83/099 473) requires solvents, such as dimethylformamide, to be used in the reaction of .gamma.-butyrolactone and ethyl acetate with sodium methylate to give 2-acetylbutyrolactone. Acid halides are also mentioned as possible condensing agents (JP 83/162 585).
The following features are characteristic of general ester condensation: the condensing agent must always be employed in at least equimolar quantities (1 mol per mole of .beta.-keto compound). The corresponding consumption of the agent is unavoidable. Furthermore, the initial product is an alkali metal salt of the .beta.-keto compound. Consequently, in principle, a salt precursor is present which must subsequently be converted, in a second step, into the desired free .beta.-keto compound. This is carried out by adding a strong acid to the salt. This step of protonation yields the free .beta.-keto compound and the alkali metal salt of the acid which was added. The individual reaction steps are illustrated below using ethyl acetoacetate as example, with the salt of the keto ester being shown in the enol form: ##STR1##
The introduction of the alkali metal ion by the condensing agent leads inevitably to the production of an alkali metal salt after protonation (Na.sub.2 SO.sub.4 in the above example). Since the quantities involved here are not catalytic but molar, the reactions unavoidably involve the production of relatively large quantities of the salt. Thus, in this reaction a valuable condensing agent is consumed and a worthless salt is produced which, furthermore, has to be disposed of.